This is a multicenter epidemiologic study to determine the impact of nontuberculous mycobacterium on the natural course of cystic fibrosis lung disease. The first aim of the study is to define the prevalence of nontuberculous mycobacterium (NTM) in geographically distributed CF centers in the U.S. Three sputum specimens will be collected for stain and culture at 3-month intervals from 1000 CF patients. Demographic and clinical features of the patients will be correlated with culture results. The second aim of the study is to define the clinical course of CF in patients with NTM. CF patients with new acquisition of NTM will be matched on a 1:2 ratio with NTM-culture negative CF patients (controls) and will be followed prospectively over a 15-month period to compare the course of pulmonary disease in the two cohorts. This is not a clinical trial, but when specific treatment is felt to be indicated by the patient's physician, a number of descriptive data will be collected.